The Normalized Difference Vegetation Index (NDVI), one of the most popular, is a valuable way to understand vegetation health and land use remotely. In this article, we tell you what it is and how to use it.
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1. What is the NDVI?
NDVI or greenness index is an indicator that shows the greenness, density and health of vegetation in each pixel of a satellite image.
It has been one of the most widely used vegetation indices in remote sensing since its introduction in the 1970s, and digital agriculture is one of the industries that takes most advantage of it.
Thus, this index is suitable for estimating vigor throughout the crop cycle based on how plants reflect certain ranges of the electromagnetic spectrum. It allows to know its current state, which can then be compared with another temporal image to observe its evolution over time.
By knowing how NDVI works, we will understand how it allows us to determine how healthy or unhealthy a plant is. This index is based on how it reflects energy and light. It uses the near infrared and red bands of the electromagnetic spectrum to estimate a dimensionless indicator between -1 and 1.
To the human eye, a plant is green because the chlorophyll pigment it contains reflects green waves and absorbs red waves. This means that a healthy plant–with lots of chlorophyll and cell structures–actively absorbs red light and reflects NIR when photosynthesis occurs. The plant develops and grows and contains more cell structures. With an unhealthy plant, the exact opposite is true.
On our platform, the scale to the right of the field is the reference for crop vigor.
So, that relationship between light and chlorophyll is how we can use NDVI to differentiate a healthy plant from a diseased one.
Satellite sensors in space measure the wavelengths of light absorbed and reflected by green
plants. They are an excellent source of spectral signature data for NDVI analysis.
The NDVI index detects and quantifies the presence of living green vegetation using this reflected light in the visible and near-infrared bands.
As for how the NDVI index is calculated, the standard NDVI formula is used
NDVI = (NIR – red) / (NIR + red)
2. How to use NDVI? How does it vary according to crop condition?
The NDVI value ranges from -1 to 1 and shows the vigor of the crop:
⦁ Values close to 1: the more intense the green, the more vigorous the vegetation and vegetation cover. We must take into account whether we are working with extensive or intensive farming, or if there is bare soil, since all this will be taken into account by the index. And it will also measure the vigor of the underbrush.
⦁ Values close to 0: correspond to areas with very little vegetation, early stages of cultivation, bare soil or non-productive areas.
⦁ Negative values: usually associated with areas of water, snow or clouds.
3. What is the GNDVI and how does it differ from NDVI?
GNDVI (Green Normalized Difference Vegetation) is an index of plant “greenness” or photosynthetic activity. It is a chlorophyll index and is used at later stages of development, as it saturates later than NDVI. It is one of the most widely used vegetation indices to determine water and nitrogen uptake in the crop canopy.
As with NDVI, the values given by this index also range from -1 to 1:
⦁ Values between -1 and 0: are associated with the presence of water or bare soil. This index is mainly used in the intermediate and final stages of the crop cycle.
GNDVI is the green vegetation index that uses the near infrared (NIR) and green band (GREEN) of the electromagnetic spectrum.
GNDVI is more sensitive to chlorophyll variation in the crop than NDVI and has a higher saturation point. It can be used in crops with dense canopies or in more advanced stages of development while NDVI is suitable for estimating crop vigor during the early stages.
As for how it is calculated, the GNDVI is the green vegetation index that uses the near infrared (NIR) and green band (GREEN) of the electromagnetic spectrum.
GNDVI = (NIR-GREEN) /(NDVI+GREEN)
4. NDVI variants: NDRE, MSAVI2, and VISIBLE
NDRE (Normalized Difference Red Edge) is an index very similar to NDVI. Its main difference lies in a safer solution, as it can detect variations in crop health at more advanced stages. This is because it uses red-edged light that can penetrate leaves much deeper than red light (used in NDVI). Thus, where there is more intense canopy it is advisable to use NDRE because NDVI saturates.
This index detects changes in chlorophyll content, which is one of the main indicators of nitrogen inside the leaves.
NDRE = (NIR – RED EDGE) / (NIR + RED EDGE)
How is the NDRE used?
NDRE is used in the advanced stages of the crop, where we can generate nitrogen fertilization maps in the most affected areas.
It is an index closely related to NDVI; however, it allows to identify vigor decreases earlier.
The MSAVI2 or Modified Soil Adjusted Vegetation Index is a vegetation index that is used as a variant to extend the application limits of NDVI to areas with a high presence of bare soil. It is used in areas where indices such as NDVI or NDRE provide incomplete or erroneous data, mainly due to a small amount of vegetation or lack of chlorophyll in the vegetation (e.g. in the phenological stage of emergence). In this way, it serves to minimize the influence of the soil background and increase the dynamic range signaled by the vegetation.
How is MSAVI2 used?
MSAVI2 is used in the early stages of crop development, where it will allow us to observe the first seedlings emerging.
It is used to minimize the influence of bare soil, so it is ideal for early stages, such as crop emergence, crops that do not cover the soil in its most developed stage or for woody crops. This index allows us to be more efficient in the use of fertilizers during the early stages reducing the environmental impact and increasing production significantly.
In addition to the NDVI and GNDVI indices, it is possible to view the raw (RGB) satellite image known as Visible in the bottom selector. It is important to view the visible image first, to corroborate that there are no clouds or overcast that may be affecting the NDVI and GNDVI indices.
5. NDWI: The water index and how to use it.
The NDWI, Normalized Difference Water Index, is used to monitor crop water status. It is used to observe the water status of the crop, identify moisture deficit and saturation in the crop. This index uses green and near infrared bands of satellite images. NDWI can improve water information efficiently in most cases. It is sensitive to soil accumulation and results in overestimation of water bodies. NDWI products can be used in conjunction with NDVI change products to assess the context of apparent change areas.
NDWI=(NIR-SWIR) / (NIR+SWIR)
How is the NDWI used?
As with other indices, the values obtained for NDWI range from -1 to 1, where high values correspond to high plant water content and coverage of a large part of the plant and low values represent low vegetation water content and sparse cover.